Energy crisis and environment pollution are two big problems for achieving sustainable development of global economics. An efficient way to solve the two problems is to develop more effective renewable energy such as wind energy, solar energy, tidal energy, etc. To ensure stable supply of renewable energy such as wind energy, solar energy, etc., an energy storage technology with high capacity, low cost, high efficiency, and high reliability and without pollution must be developed. Therefore, one of hot spots in the world in energy field is to develop an energy storage system with high capacity.
Among various energy storage systems with high capacity, vanadium redox battery (VRB) has been put into demonstrative operation in wind power generation, solar power generation, and the peak regulation of power grid and the like in foreign countries because of its unique advantages such as long lifetime, high reliability, low cost for operation and maintenance, and the like.
Vanadium battery uses solutions of vanadium ions with different valences as active species, wherein V4+/V5+ redox couple is used for positive electrode and V2+/V3+ redox couple is used for negative electrode. During a charge process, V4+ is changed into V5+ at positive electrode and V3+ is changed into V2+ at negative electrode. During a discharge process, V5 is changed into V4+ at positive electrode and V2+ is changed into V3+. A cell of vanadium battery is comprised of a bipolar plate, electrodes and a separator membrane, wherein the separator membrane of vanadium battery must be able to prevent vanadium ions of different valences in the electrolytes for the positive electrode and for the negative electrode from permeating through the separator membrane but permitting the transfer of proton of hydrogen through the separator membrane. Therefore, the separator membrane should have a desirable proton conductivity as well as a high selective permeability for protons. Furthermore, the separator membrane must have a long-term chemical stability and good mechanical properties so as to meet the long life-time requirement of vanadium battery.
Currently, the common used membrane in vanadium battery is the perfluorosulfonic acid proton exchange membrane provided by DuPont Company under the trade name of Nafion. Perfluorosulfonic acid proton exchange membrane has excellent chemical stability and ion conductivity and can meet the requirement of vanadium battery. However, Perfluorosulfonic acid proton exchange membrane has poor permselectivity and vanadium ions can permeate through the membrane during the operation of vanadium battery. Thus, the self-discharge of vanadium battery occurs and the capacity of vanadium battery is reduced. Furthermore, the high cost of perfluorosulfonic acid proton exchange membrane is one of the factors obstructing the large scale commercialization of vanadium battery. Therefore, an important step for the commercialization of vanadium battery is to develop a proton exchange membranes suitable for use in vanadium battery with low cost, high chemical stability, good ion conductivity, high permselectivity and high mechanical strength.
In the field of fuel battery, in order to reduce the cost of proton exchange membrane, some non-flouorous hydrocarbon polymers are extensively studied to be used as the membrane forming material after being sulfonated. Such kinds of polymers generally have properties such as high chemical and thermal stability, and low cost, for example, polyethersulfone, poly (ether ketone), polyimide, polyphosphazene, polybenzimidazole, etc. Theses polymers are sulfonated to form proton exchange membranes, and the resulting membranes have a property that the properties thereof such as proton conductivity of the membrane depend on a degree of sulfonation of the polymer. The degree of sulfonation of the polymer shall be high enough so that an ideal conductivity is obtained. However, when the degree of sulfonation of the polymer is high, a mechanical property and dimensional and chemical stabilities of the membrane will become poor and thus the requirement of usage will not be satisfied. The proton exchange membrane used in the vanadium battery can also be made from these sulfonated polymers. However, these membranes will face a similar problem, i.e., how to compromise among degree of sulfonation, ion conductivity and chemical stability, mechanical strength, and vanadium ion permeability.